Changelog, dev startup, README, AgentHub 0.4.0, model catalog, and landing site (#7)

Branch-length batch covering tooling, the model layer, the Web App and the public
surfaces. Highlights:

- Changelog: a per-release `changelog/<version>/` tree, grouped by the surface each
  change touches, with a root CHANGELOG.md holding one line per release.
- Dev startup: `scripts/dev-prebuild.mjs` serializes the skills+core prebuild behind a
  lock and keeps `pnpm install` current; `pnpm dev` runs server+web together.
- AgentHub 0.3.3 -> 0.4.0: OmniMessage complete payloads carry one opaque `fidelity`
  object in place of item-level `signature`/`phase`, threaded verbatim through Trace,
  replay and resume; malformed classification adapted to the new error types.
- Model layer: a model is always referenced by an explicit `(provider, model_id)` pair.
  The provider is never inferred, guessed or defaulted -- both the catalog inference and
  the unique-match config resolution are gone, and CLI, SDK, server routes and
  run_subagent all require the complete pair. Catalog gains the Qwen Token Plan, Qwen
  Pay-As-You-Go and Fireworks AI gateways, plus an expanded OpenRouter group.
- Web App: catalog preset sync and per-group speed test on the Models page, positional
  slash commands, a markdown renderer, skill-library update reminders, and a vertically
  centred draft page whose upward menus size themselves to the room available.
- Public surfaces: restructured READMEs, the penguin.ooo landing site and blog, refreshed
  benchmark results for both suites, and the demo videos playing on the landing page.

Includes the fixes from a full review of the branch: 23 confirmed findings, among them a
provider-inference bug that could send one vendor's API key to another vendor's endpoint,
and an Escape handler that destroyed the composer's contents unrecoverably.

Verified on the branch head: pnpm test (1127 passing, 7 packages), pnpm typecheck and
pnpm format:check clean, Playwright e2e 14/14.

packages/server/test/workspace-files.test.ts

@@ -122,6 +122,7 @@ describe("files/stat route (batch existence check)", () => {
   let owner: ReturnType<typeof apiClient>;
   let outsider: ReturnType<typeof apiClient>;
   let sessionId: string;
+  let workspace: string;
 
   beforeEach(async () => {
     t = await createTestApp();
@@ -141,6 +142,7 @@ describe("files/stat route (batch existence check)", () => {
       await owner.post(`/api/projects/${projectId}/agents/default_agent/sessions`, {})
     ).json()) as SessionCreateResponse;
     sessionId = sess.session.sessionId;
+    workspace = sess.session.workspace;
     await fs.mkdir(path.join(sess.session.workspace, "sub"));
     await fs.writeFile(path.join(sess.session.workspace, "a.txt"), "A");
     await fs.writeFile(path.join(sess.session.workspace, "sub", "b.md"), "B");
@@ -149,6 +151,35 @@ describe("files/stat route (batch existence check)", () => {
     await t.cleanup();
   });
 
+  it("files/content on html: inline stays text/plain; preview=1 keeps text/html under a CSP sandbox; download keeps the real type with no CSP", async () => {
+    await fs.writeFile(path.join(workspace, "page.html"), "<!doctype html><script>1</script>");
+    const url = `/api/sessions/${sessionId}/files/content?path=page.html`;
+
+    const inline = await owner.get(url);
+    expect(inline.status).toBe(200);
+    expect(inline.headers.get("content-type")).toContain("text/plain");
+    expect(inline.headers.get("content-security-policy")).toBeNull();
+
+    const preview = await owner.get(`${url}&preview=1`);
+    expect(preview.status).toBe(200);
+    expect(preview.headers.get("content-type")).toContain("text/html");
+    const csp = preview.headers.get("content-security-policy") ?? "";
+    expect(csp).toContain("sandbox");
+    expect(csp).not.toContain("allow-same-origin");
+    expect(preview.headers.get("content-disposition")).toContain("inline");
+
+    // download wins over preview: attachment + real type, no CSP needed.
+    const download = await owner.get(`${url}&download=1&preview=1`);
+    expect(download.headers.get("content-type")).toContain("text/html");
+    expect(download.headers.get("content-disposition")).toContain("attachment");
+    expect(download.headers.get("content-security-policy")).toBeNull();
+
+    // Non-scriptable files are unaffected by preview.
+    const txt = await owner.get(`/api/sessions/${sessionId}/files/content?path=a.txt&preview=1`);
+    expect(txt.headers.get("content-type")).toContain("text/plain");
+    expect(txt.headers.get("content-security-policy")).toBeNull();
+  });
+
   it("existing files return in order, deduplicated; missing / directory / out-of-bounds all count as nonexistent, always 200", async () => {
     const res = await owner.post(`/api/sessions/${sessionId}/files/stat`, {
       paths: ["sub/b.md", "a.txt", "sub/b.md", "nope.txt", "sub", "../escape.txt", "/etc/passwd"],